The present invention relates to a method of preparing 1,3-propanediol by means of the hydration of acrolein in the presence of an acidic cation exchanger followed by the subsequent catalytic hydrogenation of the 3-hydroxypropionaldehyde.
1,3-propanediol has many potential applications as a monomeric component for the production of polyesters and polyurethanes as well as an initial starting material for the synthesis of cyclic compounds. Many methods have already been suggested for the preparation of 1,3-propanediol, including those which involve a molecular synthesis from a C.sub.2 component and a C.sub.1 component and those which start from a C.sub.3 component such as acrolein in particular.
As is known from U.S. Pat. No. 2,434,110, acrolein can be hydrated in the presence of an acidic catalyst to form 3-hydroxypropionaldehyde. The reaction preferably takes place at an elevated temperature using a 5 to 30% by weight solution of acrolein in water and an acid such as for example sulfuric acid, phosphoric acid or acidic salts of these acids as the catalyst. The reaction mixture obtained during the hydration is hydrogenated, optionally after removal of nonreacted acrolein, in the presence of customary hydrogenation catalysts. Catalysts containing one or more metals active in hydrogenation reactions such as for example Fe, Co, Ni, Cu, Ag, Mo, W, V, Cr, Rh, Pd, Os, Ir and Pt are suitable as catalysts for the hydrogenation of 3-hydroxypropionaldehyde to 1,3-propanediol.
A disadvantage in the method of U.S. Pat. No. 2,434,110 are the low yields of 1,3-propanediol, which are attributable especially to acrolein--consuming condensation reactions during the hydration stage. In addition, the selectivity of the mineral acid catalyzed hydration reaction is dependent to a very large extent on the conversion of acrolein. In order to achieve an acceptable selectivity, the hydration reaction is terminated at a low acrolein conversion, which, however, results in a poor volume-time yield.
There has been no shortage of attempts to obviate the disadvantages of the above described method, for example by means of the addition of lower carboxylic acids onto the double bond of the acrolein. However, this makes a saponification step necessary after the hydrogenation. In addition, the recycling of the carboxylic acid poses problems (U.S. Pat. No. 2,638,279). The hydration of acrolein using carbon dioxide as the catalyst is also known however, this method requires long reaction times--cf. DE-OS 19 05 823.
It has been determined that although the hydration of acrolein can be carried out using for example phosphoric acid or dihydrogen phosphates catalyst but problems occur in the subsequent hydrogenation of the reaction mixture freed from non-reacted acrolein.
When very active nickel hydrogenation catalysts are used, both the hydrogenation conversion and also the reaction speed drop rapidly upon repeated use of the catalyst. This leads to an elevated consumption of catalyst.
In addition, the presence of the hydration catalyst during working up by distillation results in product losses due to decomposition or, in the case of first carrying out a neutralization, results in cloggings and encrustations in the system. The problems indicated above can be partly obviated if the hydration catalyst is removed from the reaction mixture before hydrogenation by means of ion exchange resins or by separating 3-hydroxypropionaldehyde from the reaction mixture and then subjecting it to hydrogenation. However, both alternative measures for reducing the consumption of expensive hydrogenation catalyst necessitate additional equipment, lead to a higher consumption of energy and, additionally, in wastewater problems, thus increasing the production costs for 1,3-propanediol.
According to the method of U.S. Pat. No. 3,536,763, the hydration step is carried out in the presence of weakly acidic cation exchanger resins the functional groups of which are carboxyl groups at 40.degree. to 120.degree. C. Preferably, 0.1 to 5% of the functional groups should be present in the form of an alkali carboxylate, alkaline earth carboxylate or earth metal carboxylate. The yields of 3-hydroxypropionaldehyde are indicated to be approximately 80%, and are said to be substantially independent of the acrolein conversion in the range of 25 to 65%. This process also comprises the known hydrogenation of 3-hydroxypropionaldehyde to 1,3-propanediol.
Although the catalytic activity of the ion exchanger resins with carboxyl groups was confirmed when the method of U.S. Pat. No. 3,536,763 was followed, the level of effectiveness suggested that these ion exchangers would not be suitable for use in industrial systems. It turned out that these catalysts require rather high temperatures and rather long reaction times, which runs counter to the desired high volume-time yield.